Electronic excitation energy relaxation

Thus, the electronic excitation energy is first spent on a chemical bond breaking (the relaxation of the electronically excited site) and, then, a certain part of the energy is released in the form of the heat during the reconstruction of the disrupted bond. The model proposed may explain the high photo stability of the E -centers in Si02. 

S. Comi, R. Cammi, B. Mennucci, J. Tomasi, Electronic excitation energies of molecules in solution within continuum solvation models Investigating the discrepancy between state-specific and linear-response methods, Formation and relaxation of excited states in solution A new time dependent polarizable continuum model based on time dependent density functional theory. J. Chem. Phys. 123, 134512 (2005)... 

The ultrafast PT, which occurs typically on time scales of 10 13-10 14 s will not be considered. Such transfers are observed in molecular systems in which the potential energy surface (PES) governing the proton motion is essentially barrierless but has different minima positions in different electronic states, so that the proton finds itself in an off-equilibrium position after electronic excitation and relaxes to the new equilibrium position. The contribution of tunneling may be disregarded and the rate of these processes does not depend very strongly on temperature. These reactions, which are of great current interest, are intensely studied by ultrafast laser spectroscopy and are reviewed elsewhere [16,17],... 

Therefore, a single SCF calculation is sufficient. Many investigations have shown that, for the large majority of organic compounds, this approximation can be applied. The success of Koopmans theorem is due to the fact that electron relaxation effects as well as correlation effects nearly cancel each other. Nevertheless, it must be emphasized that MOs cannot be observed they are one-electron wave functions which merely serve as convenient model building blocks in the theoretical construction of molecular many-electron states. It should also be kept in mind that this simple picture may occasionally break down, particularly for compounds with small electronic excitation energies . ... 

The low fluorescence quantum yield and the fact that no phosphorescence could be observed in oligothio-phenes leads to the conclusion that most of the electronic excitation energy decays radiationless. There are different views of the participation of triplet states in this decay process in thin films. In solution, however, the relaxation via triplet states is well agreed. Tliis is due to the fact that singlet oxygen is produced and the absorption attributed to triplet-triplet excitation (i.e. at 1.80 eV [139] respectively 1.92 eV [38], theoretically predicted value around 2.7 eV [172]) is completely quenched by exposure to air. 

The lifetime of an analyte in the excited state. A, is short typically 10 -10 s for electronic excited states and 10 s for vibrational excited states. Relaxation occurs through collisions between A and other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational deactivation, or nonradiative relaxation, the excess energy is released as heat thus... 

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